1. Field of the Invention
The present invention relates to a resin molded product, especially a fuel tank.
2. Description of the Related Art
Resin fuel tanks for automobiles are classified into two types. In the first type, a fuel tank is integrally molded by blow molding. In the second type, two split molded parts are formed by injection molding and welded at a welding portion to thereby form a fuel tank.
(1) In the fuel tank integrally molded by blow molding, a high-density polyethylene (hereinafter abbreviated as HDPE) resin excellent in impact resistance and chemical resistance and inexpensive has been heretofore often used as a resin material of a parison. There is however a problem that the HDPE resin is high in fuel permeability. Therefore, a multilayer structure including a barrier layer made of an ethylene-vinyl alcohol (hereinafter abbreviated as EVOH) copolymer resin low in fuel permeability has been examined.
Unexamined Japanese Patent Publication No. Hei-6-340033 has disclosed a fuel tank as a multilayer structure having a polyolefin layer treated with a halogen compound or a sulfur compound, and a resin layer containing an EVOH copolymer or polyamide. FIGS. 2A and 2B show an applied example of the fuel tank. In FIGS. 2A and 2B, the fuel tank 60 is formed by blow molding of a parison 50 having a four-material six-layer structure as a laminate of an HDPE resin layer 53, an adhesive layer 52, a barrier layer 51 made of an EVOH copolymer, an adhesive layer 52, a regenerated resin layer 54, and an HDPE resin layer 53.
Unexamined Japanese Patent Publication No. Hei-6-191296 has disclosed an automobile fuel tank made of a multilayer hollow molded product having a laminated structure composed of a barrier layer containing an aromatic polyamide-Nylon-6 copolymeric polyamide resin as a main component, and inner and outer layers each containing an HDPE resin as a main component and bonded to both surfaces of the barrier layer respectively through adhesive layers each containing a modified HDPE resin as a main component.
(2) On the other hand, in the fuel tank formed in such a manner that two split molded parts formed by injection molding are welded at a welding portion, Nylon-6 and Nylon-12 are used as injection molding materials as disclosed in Japanese Utility Model Publication No. Sho-62-20922.
The fuel tank formed by blow molding as in (1) is inferior in production efficiency. Moreover, in the fuel tank (FIGS. 2A and 2B) disclosed in the above JP Hei-6-340033, parts of the barrier layer 51 are departed from each other so as to be discontinuous to each other because parts of the innermost HDPE resin layer 53 fuse at a pinch portion 55 for pinching a terminal at the time of blow molding. Because the pinch portion 55 is located in a place where fuel comes into contact with the inside of the pinch portion 55, there is a problem that (a small amount of) fuel is transmitted easily through the departing portion of the barrier layer 51. In addition, when parts such as a pump, a valve, etc. need to be mounted in the fuel tank, there are required a process of drilling a mount hole 56 in a general portion of the fuel tank, a process of tightening the parts through a gasket and a process of performing hot plate welding of PE resins. These processes cause a disadvantage in terms of transmission of fuel. These problems occur also in the fuel tank disclosed in the above JP Hei-6-191296.
In the fuel tank formed by injection molding and welding as in (2), the Nylon resins are low in barrier characteristic to fuel containing alcohol such as methanol or ethanol. There is a problem that a large amount of fuel is transmitted through the tank as a whole.
Therefore, the present inventors have tried injection-molding a fuel tank by using a composition containing a polyphenylene sulfide (hereinafter abbreviated as PPS) resin high in barrier characteristic at the initial stage of development of the invention. It has been however found that the PPS resin composition which has been ever developed has insufficient fluidity when it is used as a material for injection-molding a large-size molded product such as a fuel tank. The situation in which the PPS resin composition has been heretofore developed will be described below in brief.
The PPS resin has excellent properties adapted to engineering plastics, such as heat resistance, barrier characteristic, chemical resistance, electrical insulating characteristic, humidity resistance, heat resistance, etc. The PPS resin is mainly used for the purpose of injection-molding various kinds of electric/electronic parts, mechanical parts and automobile parts. The PPS resin is however lower in flexibility than other engineering plastics such as Nylon, PBT, etc. In the existing circumstances, the application of the PPS resin is limited. Improvement of the PPS resin is required earnestly.
Melt-bending of various kinds of soft resins etc. has been examined as a method of improving flexibility of the PPS resin.
For example, Unexamined Japanese Patent Publication No. Sho-59-167040 has disclosed a method of adding a hydrogenated SBR copolymer, and Unexamined Japanese Patent No. Sho-56-115355 has disclosed a method of adding a dicarboxylic anhydrate hydrogenated SBR copolymer. These SBR copolymers are however low in miscibility with the PPS resin, so that the flexibility improving effect cannot be obtained sufficiently.
Unexamined Japanese Patent Publication No. Sho-61-21156 has disclosed a method of adding an α-olefin-glycidyl methacrylate copolymer. In the invention described in the above JP Sho-61-21156, there has been not obtained any composition having an Izod impact strength of not lower than 700 J/m.
Unexamined Japanese Patent Publication No. Hei-4-159364 and Hei-4-159365 have disclosed a composition having a melt viscosity of not lower than a predetermined value and containing a PPS resin having a specific terminal group, and an olefin copolymer formed from α-olefin and α,β-unsaturated acid glycidyl ester. Also in the inventions described in the above JP Hei-4-159364 and Hei-4-159365, there has been not obtained any composition having a tensile elongation of not lower than 50% at break.
Unexamined Japanese Patent Publications No. Hei-6-179791 and Hei-6-299071 have disclosed a composition containing 70% by weight to 99.5% by weight of a mixture of α-olefin (10% by weight to 50% by weight) and α,β-unsaturated carboxylic alkyl ester (50% by weight to 90% by weight), and 0.5% by weight to 30% by weight of α,β-unsaturated acid glycidyl ester. Also in the above JP Hei-6-179791 and Hei-6-299071, there has been not obtained any composition having an Izod impact strength of not lower than 700 J/m.
On the other hand, Unexamined Japanese Patent Publications No. Hei-1-306467, No. Hei-3-68656 and No. 2000-198923 have succeeded in achieving high flexibility and high impact resistance simultaneously by mixing a PPS resin having a specific structure and an olefin copolymer at a predetermined ratio. Each composition capable of achieving very high impact resistance of not lower than 700 J/m, however, needs to contain an excessively large amount of a soft resin component. For this reason, there is a disadvantage in that heat resistance and chemical resistance as original characteristic of the PPS resin are worsened.
Unexamined Japanese Patent Publication No. Hei-11-100506 has disclosed a method in which at least one kind of anti-oxidizing agent selected from the group consisting of a phenol anti-oxidizing agent, a thioether anti-oxidizing agent and a phosphorus anti-oxidizing agent is added to an olefin copolymer to suppress thermal deterioration at the time of kneading or molding to thereby improve toughness. In the above JP Hei-11-100506, there has been not obtained any composition having an Izod impact strength of not lower than 700 J/m.
Therefore, a PPS resin excellent in impact resistance and relatively excellent in moldability such as fluidity has been proposed as follows.
Unexamined Japanese Patent Publication No. Hei-4-24388 has disclosed a composition containing a PPS resin, an epoxy group-containing olefin copolymer, and an elastomer containing neither epoxy group nor acid anhydrate group.
Unexamined Japanese Patent Publication No. 2002-226604 has disclosed a composition containing a PPS resin and an olefin resin and having a tensile elongation of not lower than 20% at break.
Unexamined Japanese Patent Publication No. 2002-226706 has disclosed a composition containing a PPS resin and an olefin resin and having an Izod impact strength of not lower than 700 J/m.
Unexamined Japanese Patent Publication No. 2002-226707 has disclosed parts of a fuel system, containing a PPS resin and an olefin resin and having a fuel transmission coefficient of not higher than 3.3×10−16 mol·m/m2·s·Pa.
Particularly in the above JP 2002-226707, “a fuel tank, a valve, a fuel hose, a fuel hose joint, a fuel pump, a fuel pump housing, and a canister” have been shown as specific examples of parts of the fuel system. It is certainly possible to mold these specific examples from the PPS resin composition according to JP 2002-226707. According to the inventors' further examination of the PPS resin composition, it has been however found that the fuel tank formed from the PPS resin composition by injection molding lacks fluidity as described above. That is, high fluidity is required of the fuel tank because the fuel tank is a molded product which is relatively large in size but not thick (even in the case where the fuel tank is divided into two parts). The inventors have tried forming the fuel tank from the resin composition containing the PPS resin according to JP 2002-226707 by injection-molding (formation of two split parts). As a result, there is however a possibility that underfill may occur because the molding die cannot be filled with the resin composition.